The importance of assuring that compressed air is as clean and dry as possible before using it in any type of application is universally appreciated. The process of compressing air not only concentrates impurities in the atmosphere, but adds many more, rendering the final product totally unacceptable for many of the very wide range of applications involving the use of compressed air.
There are many divergent methods available to remove contaminates from compressed air and most of them consist of passing the air through a barrier. The barrier may be chemical or mechanical, but it will always have a limited period of effectiveness, the period being governed by the time it takes to become saturated with contaminates. When this happens the barrier must be cleaned or replaced, involving maintenance time and costs, or the contaminates will be forced into the air line. Anything which will prolong the effectiveness of the barrier is not only a cost saver but also an assurance of a more consistent supply of properly filtered air.
The most basic and effective method of removing the bulk of contaminates carried in an air flow is the use of a drop leg. This strictly mechanical device never clogs up, never wears out and, apart from having to have the accumulation of contaminates drained off, requires no maintenance. It uses a natural phenomena of air flow to separate gas and contaminate molecules and the greater the flow, the more efficient it becomes.
Drop legs have been in use since the first compressed air lines were installed. They originated in a similar capacity for steam lines and were converted to use with compressed air without any major changes. Their effectiveness is qualified by the fact that most suppliers of pneumatic equipment, including filters and dryers, recommend that a drop leg be installed prior to their equipment.
A conventional drop leg consists of a single tube carried past the point of take off of an air drop, an air drop merely being the pipe which carries the air down to working level from the overhead main pipe, or "header." The diameter of the down pipe is usually expanded below the take-off point, capped and fitted with a drain cork. These legs function due to the fact that air molecules travel in a straight line and, having been directed into a flow path by the down pipe, continue in that path until they impinge on the cap at the base of the tube. The gas molecules rebound and continue to move with undiminished energy, but anything which has no energy of its own, usually such as water, dirt and oil, is left at the point of impingement.
This is, of course, a "catch as catch can" arrangement and rapidly loses its original efficiency when any accumulation of contaminates occurs. The turbulence then actually re-introduces the lighter contaminate matter into the air flow, leaving only the heavier matter at the base of the tube. This is evident when draining a drop leg and a heavy sludge is drawn off. Nevertheless, this is the very stuff which is most dangerous to filter screens and tools so the leg serves a very worthwhile purpose.
It has been proposed to provide drop legs with a separate air outlet. As far as we are aware, such air outlets have required the air, after striking the impingement surface to pass downwardly below that surface in a turbulent manner such as to cause reentry into the outlet air stream of at least the lighter contaminants which may have been removed. See U.S. Pat. Nos. 2,467,408 and 3,345,807.